For the same reason $500 laptops with 4GB RAM come with Windows 7 64-bit. With Mac's and PC's selling with 4GB memory and up, Apple and Microsoft are trying to tell us it's time to move to 64-bit. Yes the Mac Pro comes with 3GB RAM, but that is a marketing decision that I don't understand - when a 13-inch MacBook Pro comes with 4GB, why doesn't the Mac Pro come with 6GB? Anyway, with many Mac Pro's soon to be running with 24GB and 32GB of memory, the 64-bit kernel should be the default.

The 2009 Mac Pro boots in 64-bit mode by default if you install OS X Server

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The 2010 Mac Mini Server with OS X Server also boots into 64-bit mode by default. The only problem I had: MacFuse to read/write NTFS disks only works with the 32-bit kernel. There is a hacked 64-bit MacFuse but I can't trust my data to that, so I bought Paragon NTFS for Mac which is 64-bit.

I benchmarked my '09 Pro with both the 32 and 64bit kernel and had the exact same results.

IMHO the only opinion to use the 64bit kernel is if you have more than 32GB of RAM installed, performance wise it doesn't make a difference (or maybe I just used the wrong benchmarks or software that doesn't stack up with the hardware ).

Because it can. Why shouldn't it? There comes a really minor performance improvement with it.
On Macs, it doesn't bring that much of improvement, as most stuff already runs 64Bit on 32Bit-Kernel, i.e. drivers, as the Kernel runs in the kernel runs in 64Bit Compatibility-Mode, so actually the CPU is already in 64Bit (aka long)-mode rather then being completely 32Bit.

PenguinMac said:

For the same reason $500 laptops with 4GB RAM come with Windows 7 64-bit. With Mac's and PC's selling with 4GB memory and up, Apple and Microsoft are trying to tell us it's time to move to 64-bit. Yes the Mac Pro comes with 3GB RAM, but that is a marketing decision that I don't understand - when a 13-inch MacBook Pro comes with 4GB, why doesn't the Mac Pro come with 6GB? Anyway, with many Mac Pro's soon to be running with 24GB and 32GB of memory, the 64-bit kernel should be the default.

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BS. Even with a 32Bit-Kernel, the CPU runs in long-mode, thus can adress much more then even the 64GB of PAE, as the long-mode is a superset of PAE.

so what does this mean now for 2010 mac pro owners?? Does this mean certain software won't be able to run on these new mac pros. They talked about drivers, drivers for what exactly, but I don't really think I understand all of this.

but that is a marketing decision that I don't understand - when a 13-inch MacBook Pro comes with 4GB, why doesn't the Mac Pro come with 6GB?

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Probably because it isn't just a pure marketing decision (or is marketing decision ... depends if you think the marketing folks care about quality or not.)

Mac Pro and MBP are two different market segments. Mac Pro users typically want to open the box and tweak the configuration. In that context, the user is going to change the configuration anyway after they get it. It is not a "buy and then use for months/years before change" situation. It is a "buy it and change after a day or two. " situation. If the user doesn't really want Apple's RAM then it is better to sell the cheapest RAM possible so no big deal with them putting on the shelf (or trading in) when they buy the RAM config the want from someone else. That way the can also crow about how they saved bucks but not paying Apple "sky high" RAM fees.

For those who don't mind paying apple prices for RAM it isn't just 3GB you can get it in. Typically those folks put value on one purchase order and configuration support over saves a few bucks. Nobody "has to" buy a 3GB Mac Pro from Apple. There are more memory configurations you can buy in the Apple store for the Mac Pro than the MBP.

MPB: 2x different flavors of DIMMs on menu.
MP: 3x and 4x different flavors of DIMMs on menu.

The 1GB flavor boxes are Apple's version of a "bare bones" box offering.

As for quality there are three different memory channels. A simplistic QA check at manufacturing would be to put in something to test all three when do the boot smoke test before boxing. So stuff something into all slots with a distinct channel. The MBPs generally have have two channels and the Mac Pro has three. [ won't be too surprised in the 4x channel next gen Mac Pro's ship with 4x 1GB DIMMs . Would be natural for Apple to leverage the memory decrease over time costs to jump to 2GB/slot, but since the number of slots to be filled will likely have gone up... they are reluctant too... because pinching pennies. ]

The MBP happens to get use 2GB sticks. That's more so because Apple would rather you do not open your MBP to muck around with updates. You can, but they'd rather you not. There are also fewer slot to fill.

so what does this mean now for 2010 mac pro owners?? Does this mean certain software won't be able to run on these new mac pros. They talked about drivers, drivers for what exactly, but I don't really think I understand all of this.

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Perhiperals like Firewire, USB, PCI-e devices sometimes comes with pieces of software that "drive" the communication between the Mac and the add-on device. For example printers have drivers.

Some drivers are written by Apple. Typically those are generic. The more non generic funcitionality your add-on device has the more likely it needs a special driver. Another set of drivers are collected by Apple and distributed with system updates. Apple just ships, not writes them.

The crux of the matter drivers generally have to tightly integrate with the OS kernel. So if make major change to kernel ( go from 32 to 64 bits ) you have to change the drivers.

What will happen with 64 bit kernel is that potentially printer, PCI-e , etc. will stop working because that company hasn't released updated 64 bit versions of their drivers. That's not really a big issue at this point. With the Snow Leopard release most manufacturers got on board with 64 kernels for their then current offerings.

The big problem tends to be someone who is pushing old legacy cards/printers/etc. that aren't supported anymore. The 64-bit kernel will likely send them over the edge into not working mode. They'll need to replace or just simply boot 32-bit all the time.

Folks would bought highly specialized, low volume equipment are usually more freaked out about this. If you attach relatively modern and mainstream stuff to your Mac you probably have no issues with either kernel version.

The vast majority of Applications just "talk" to the OS. The don't require kernel modifications. The 32-bit kernel can run the more common 64-bit apps (all other macs ship exactly in that mode) and the 64-bit kernel can run 32 and 64 bit apps transparently. So no big impact at the app level. As long as they don't dip into the kernels internals the mode of the kernel shouldn't really matter.

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